Method for selectively etching alxga1-xas multiplier structures

ABSTRACT

A method for selectively etching a layer of AlxGa1 xAs with a low concentration of Al in a multilayer structure while leaving adjacent layers of AlxGa1 xAs material of higher concentration Al relatively unaffected. The structure is immersed in an etchant consisting essentially of an H2O2 solution neutralized to a pH of 6-8 with a suitable hydroxide such as NH4OH. Preferably, the solution is agitated to insure that the etched surfaces are smooth. This selective etching technique may be used to form a variety of structures including a passive optical waveguide of extremely small dimensions.

wsaaacm Qiialfitiij fit? [75} lnventors: John Cameron Dymem, Chatham;

Ralph Andre Logan, Morristown; Bertram Schwartz, Westfield, all of NJ.

[7 Assignee: Beil Telephone Laboratories,

Incorporated, Murray Hill, NJ.

221 Filed: Sept.25,1972

[Zl] Appl.N0.:291,941

[56] References Cited UNITED STATES PATENTS 3,293,092 12/1966 Gurm 156/17 I Dy merit et al. 5 Apr. 7 3974 METHOD FOR SELECTWELY ETCi-ll'e/G- 3,730,799 5/1973 Scour-ell 156/5 5frill,F l 5????53??? 3 Primary Examiner---Nilliam A. Powell Assistant Examiner-Brian J. Leitten zl/Zorney, Agent, or Firrn--L. H. Birnbaum 57 ABSTRACT A method for selectively etching a layer of Al Ga As with slow concentration of Al in a multilayer 9 Claims, 3 @rawing Figures BACKGROUND OF THE INVENTION This invention relates to a method for selectively etching certain layers in an Al Ga As multilayer structure.

Multilayer structures employing Al Ga, As where I 0.7 have realized great significance in the field of solid state laser technology. For example, it was recently-discovered that injection lasers could be fabricated which are capable of continuous operation at room temperature. The device, known as a double heterostructure laser, comprises a layer of n or p type GaAs sandwiched between a layer of n-type AlGaAs and a layer of ptype AlGaAs with a region of n-type GaAs bounding the layer of n-type AlGaAs on the opposite surface. Light emission and'electron injection in such a device are confined to the thin layer of GaAs, resulting in a sharp reduction in current density required for lasing. (See, for example, Panish and I-layashi A New Class of Diode Lasers, Scientific American, Vol. 224, No. I, pp. 32-40 (July l97l Recently a similar structure has been proposed for use as a passive waveguide (see US. patent application of R. A. LogamB. Schwartzd. C. Tracy, Jr.-W. Wiegmann Case l8l7-l-6, filed on an even date herewith). The waveguide in one embodiment is ofthe double heteros tructurc type, comprising a layer of n-type GaAs sandwichcd between a layer of an n-type AlGaAs and a layer of p-type AlGaAs. The layer of n-type AlGaAs is grown on a substrate of n-type GaAs. The layers are etched photolithographically to form a mesa structure for propagation of low order modes along the guide. One of the major advantages of this structure is its adaptability to an ir jegrated optical system which may consist, for example, of afiiii'ii'irfpssse modulator, waveguide and detector which are formed simultaneously.

Whether the multilayer structure is utilized as an active device or a passive waveguide, it is desirable to keep the dimensions of the GaAs region as small as possible. In the active device in forward bias operation, the small dimension allows operation at high current densities with low total current flow and therefore longer lifetimes. The reduced area also minimizes device capacitance permitting operation in higher frequency circuits. In the passive waveguide, a small area permits propagation of low order or even single order modes along the guide. Although the thickness dimension may be very small as the result of liquid phase epitaxial grow th, the width dimension is not so easily controlled. (itching a mesa structure alone may not be adequate since the width dimension is limited by the definition possible through photolithographic techniques. More- \)\Ll', it is often desirable to reduce the GaAs region to dimensions which make subsequent contacting virtu- .ill impossible on a mesa structure.

li is therefore the primary object of the invention to,

These and other objects of the invention are achieved by providing a method for differentially etching layer of GaAs or AlGaAs with small concentrations of uluminum in Al Ga, ,As multilayer structures. The struc. ture is immersed in an etchant consisting essentially of an H O solution adjusted to a pH of 68 by a source of hydroxyl ions such as NH OH. In a preferred embodiment, the solution is agitated so as to produce smooth etched surfaces. Utilizing this process, it is possible to narrow a GaAs layer to a width of ill, while ill allowing contact to the structure.

BRIEF DESCRIPTION OF THE DRAWING These and other features of the invention will be delineated in detail in the description to follow. In the drawing:

FIGS. 1A and 1B are respectively cross-sectional views of an Al Ga, As multilayer structure before and after treatment in accordance with one embodiment of the invention; and I FIG. 2 is a graph of etching rate of Al Ga, As layers as a function of aluminum concentration in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION The invention is described with reference to the Al, Ga, As multilayer structure'shown in FIGS. IA-IB. It will be obvious to those skilled in the art that the process described herein may be utilized on many other Al Ga, As multilayer structures to produce any of a variety of geometries.

The device shown in FIG. 1A comprises a substrate of n-type GaAs, 10, upon which is grown a layer of ntype AlGaAs, l1. Grown thereon is a layer of n-type GaAs, t2, and formed on the GaAs layer is a layer of ptype AlGaAs, 13. All three layers may be formed by standard liquid phase or molecular beam epitaxy techniques. The layers are etched to define a mesa structure as shown. Since the fabrication of this structure forms no part of the present'invention, a detailed description thereofis omitted for the sake of brevity. (See U.S. patent application of R. A. Logan et al., supra.) Assuming-for example, that the GaAs layer will be used as zuva egyide the thickness of the layer is of the order of In and the width approximatelySu. The Al- GaAs layers, which will confine the radiation in the guide, will be approximately Zn in thickness.

As discussed previously, it is desirable to narrow the area of the GaAs region to a dimension which is not usually compatible with photolitnographic techniques and subsequent contacting. In accordance .with the invention, therefore, the layer of GaAs is selectively etched while the layers of AlGaAs are relatively unaffected.

The device was immersed in an aqueous solution of 30 percent by weight H 0 which had been adjusted to a pH of 7 by the addition of approximately 1 ml concentrated NH ,OH to 700 ml of solution. The concentration of 30 percent is convenient since this solution is commercially available. However, the concentration of H 0 may be in the range of 10-70 percent. The adjustment of pH may also be made by other sources of hydroxyl ions. Nl'l OH is preferred since it contains no cation which may contaminate the GaAs material. The pH of the solution should lie within the range of 68 otherwise the GaAs-AlGaAs interface will not be sufficiently delineated. Etching of pure GaAs will occur at the rate of approximately lu/hour in a stagnant etch.

However, if the solution is agitated in some way a rate of approxir'nately 6/.L/l1LJUl' is realized and smoother etched surfaces are produced. Apparently, thin oxide.

sheets form in a stagnant etch, while agitation flushes this oxide away. Moreover, the oxide sheets that form on the surface in a stagnant etch cause the etching rate to decrease with time ofetching. Agitation maybe performed by a variety of means such as stirring or by rotating the device in the solution in this example. the structure was placed near the periphery of a 2-inch diameter circular quartz disc in the solution with the iongitudinal dimension of the mesas normal to a diameter. During the etch, the disc was rotated in an inclined beaker containing the etchant at approximately 60 RPM.

The resulting structure is shown in FIG. 18. After about 20 minutes in the etching solution (with agitation) the region of GaAs, 12, is reduced to a width of just 1 .1., while the adjacent layers of AlGaAs are essentially unaffected. The guide is now a ing singl order mode Edition.- contact canhEiWdi'Tzibricated without the need for masking. Since the p-n junction (between layers 12 and 13) is now recessed from the edge, the metal can be evaporated over the entire surface area of the mesa without shorting the junction.

In the discussion of the invention to this point, only the etching ofa pure GaAs region in a multilayer structure has been mentioned. However, it should be noted that often the active area of a device contains small concentrations of aluminum and the present invention is equally applicable to such structures. Of course, the rate of etching will be a function of the concentration ofAl in the layer. This is illustrated in the graph of FIG. 2. Double heterostructure lasers with varying concentrations of aluminum in the p-type active regions were placed in the etching solution in accordance with the invention, without agitation, and removed after 15 minutes. The degree of etching of each region was estimated from' scanning electron microscope photographs and the etching rate in A/min was plotted on a semilog scale as a function of aluminum concentration in the active region. lt will be noticed immediately that the etching rate is not a linear function of Al concentration, showing a greater dependence at lower concentrations and remaining fairly constant for concentrations greater than .t 0.25. The maximum Al concentration desired in an active region is X =02. Within this range, it is possible to design a host of multilayer structures wherein the Al concentration of the layer to be etched differs from the adjacent layers of higher concentration Al by an amount which will give a desired differential etching in accordance withthe contemplated use ofthe device. For most applications, a differential etch rate of at least lO-l is preferable, which means that the difference between the aluminum concentration of the layen to be etched and the adjacent layer must be at minimum approximately at 0.085. However. in some instances a differential etch rate as low as two-to-one maybe adequate and, thus. the minimum difference in aluminum concentration contemplated by this invention is approximately Ax .02.

All etching described herein has been performed at room temperature. It should be clear, however, that heating the solution will cause an increase in the etch rate while maintaining the differential etch characteristics of the system.

Various additional modifications ofthe' invention will become apparent to those skilled in the art. All such variations which basically rely on the teachings through which the invention has advanced the art are properly considered within the spirit and scope of the invention.

What is claimed is:

1. On a multilayer structure comprising a first layer comprising Al Ga, As adjacent to a second layer comprising Al Ga, As wherein x x,, the method of selectively etching the first layer comprising the step of immersing said structure inan etching solution consisting essentially of an aqtieous solution of H 0 and a source of hydroxyl ions, said solution having a pH in the range 6-8.

2. The method according to claim 1 wherein x, lies within the range 0 0.2.

3. The method according to claim 1 wherein x x,

4. The method according to claim 1 wherein x x,

5. The method according to claim 1 wherein the concentration of H 0 is within the range 10-70 percent by e g t 6. The method according to claim 1 wherein the con centration of H 0 is 30 percent by weight.

7. The method according to claim 1 wherein the source of hydroxyl ions is NH OH.

8. The method according to claim 1 further comprising the step of agitating the solution while the device is immersed therein.

9. The method according to claim 8 wherein the solution is agitated by rotating the device while immersed is said solution.

Patent 0 3 801;, 391 I Dated I April 2 a 97 i John C. Dvment Ralph A. Logan, and Bertram Schwartz it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Cover page, change title from "Method for Selectively Etching Al Ga As Multiplier Structures" to I v-Method for Selectively Etching Al Ga As Multilayer Structures--.

Column 1, change title from "Method for Selectively Etching AlGa 09 As Multiplier Structures v to h -:-Method for Selectively Etching Al Ga As Y Multilayer Structures--.

Signed-and sealed this 10th day of September 1974.

ttest: v

McCOY Ma, GIBSON JR MAR HA DANN -ttesting Officer Commissioner of Patents= 

2. The method according to claim 1 wherein x1 lies within the range 0 - 0.2.
 3. The method according to claim 1 wherein x2 - x1 > or = 0.02.
 4. The method according to claim 1 wherein x2 - x1 > or = 0.085.
 5. The method according to claim 1 wherein the concentration of H2O2 is within the range 10-70 percent by weight.
 6. The method according to claim 1 wherein the concentration of H2O2 is 30 percent by weight.
 7. The method according to claim 1 wherein the source of hydroxyl ions is NH4OH.
 8. The method according to claim 1 further comprising the step of agitating the solution while the device is immersed therein.
 9. The method according to claim 8 wherein the solution is agitated by rotating the device while immersed is said solution. 